The invention relates to the preparation of a cationically modified (meth)acrylamide polymer by the Mannich reaction. The invention additionally relates to the use of the polymer as a flocculation, retention and dewatering agent.
A known process for preparing cationic polymers of a high molecular weight is to modify copolymers of poly(meth)acrylamide or (meth)acrylamide by the Mannich reaction, wherein the modification is carried out using a secondary amine and an aldehyde, typically formaldehyde, or reaction products of these. When the Mannich reaction is used in the preparation of cationic polymers, the modification is often carried out in an aqueous solution of the polymer. For example, the polymers needed in the treatment of waste waters are polymers of a very high molecular weight, and therefore the treatment must be carried out in very dilute aqueous solutions, typically less than five per cent aqueous solutions. The transport of dilute solutions over long distances is uneconomical, and additionally Mannich-treated (meth)acrylamide polymer solutions are known to be unstable, which is observable as an increase of viscosity with time.
Means of avoiding the transportation of dilute solutions include carrying out the Mannich treatment so that the (meth)acrylamide polymer is in the form of an aqueous solution emulsified in a water-insoluble solvent by means of surface active agents. The other treatment chemicals are then added to this emulsion, which may be more concentrated than the polymer solution, typically 10-40%. Thus the transportation of a dilute polymer solution to the point of use of the cationic polymer can be avoided. At the point of use, water is added to the emulsion, whereupon the polymer in the emulsion will dissolve. However, the method has the disadvantage that it is difficult to separate the solvents and surface active agents present in the emulsion from the solution to be used, and thus they will pass to the target of use in the solution, thus causing process problems and environmental problems.
The disadvantages of the solvents and surface active agents present in the emulsion polymer can be avoided only by using aqueous solutions. However, in order to avoid the transportation of dilute solutions to the point of use, it is preferable to transport the components needed in the reaction to the point of use of the cationic polymer and to carry out the Mannich treatment there.
The simplest method is first to dissolve the required (meth)acrylamide polymer in water and then to add the secondary amine and the formaldehyde at a suitable temperature, and to allow the reaction mixture to react for a suitable time. This has the disadvantage that two different chemicals are required for the modification of the polymer. Furthermore, the formaldehyde and the amines used, such as dimethylamine, are substances difficult to handle, causing, for example, odor problems and a risk of ignition. In addition, it is often necessary to carry out the treatment in a separate container, to which the dissolved polymer must be transferred for the treatment. This increases the treatment time and the number of treatment steps.
In CA patent publication 1 031 096 (G. Sackman et al.), an attempt is made to solve the problems of handling difficult chemicals by using amines which boil at higher temperatures than do simple dialkylamines. However, such amines are less reactive and less economical to use than simpler dialkylamines.
FI patent publication 62846 (Nalco Chemical Co.) proposes as a solution to the problem the Mannich reaction as a continuous process treatment by means of which it is possible to avoid unnecessary transfers between the polymer dissolution apparatus and the treatment vessel. However, the problem of handling two difficult chemicals is not avoided.
The handling of difficult chemicals can be avoided by using the mixture disclosed in U.S. Pat. No. 3,367,918 (The Dow Chemical Co.), which contains, mixed, all the components required in the Mannich treatment, such as a solid salt of a secondary amine, paraformaldehyde as a formaldehyde-producing substance, a solid polyacrylamide, and sodium carbonate as a component which raises the pH. When the mixture is dissolved in water, the Mannich reaction occurs. However, amine salts and solid high-boiling secondary amines are highly hygroscopic. In mixtures with polyacrylamide, their hygroscopicity causes adhesion of the particles in the mixture, a factor which makes the mixed powders difficult to handle.
The separate handling of secondary amine and formaldehyde is avoided if the Mannich treatment is carried out using their reaction product, dialkylaminomethanol, the handling of which is not as difficult as that of secondary amines and formaldehyde. Furthermore, the use of the reaction product reduces the number of the chemicals required for the Mannich treatment from two to one. The preparation of such a reaction product and its use together with acrylamide polymers is described in, for example, U.S. Pat. No. 2,328,901 (Grimm et al.), U.S. Pat. No. 4,010,131 (Philips et al.), U.S. Pat. No. 4,166,828 (McDonald) and U.S. Pat. No. 4,288,390 (McDonald). However, in EP patent 210 784 (Farrar et al.) it is noted that such a reaction product is unstable, for which reason it cannot be stored for long periods of time. During storage the reactivity of the reaction product is at the same time lowered.
As a result of the present invention, a process has now been achieved by which the stability of the reaction product of a secondary amine and an aldehyde can be improved significantly. The process also provides the further advantage that the cation exchange capacity of a cationically modified (meth)acrylamide polymer can easily be adjusted; this need for modifying the cation exchange capacity is very necessary in particular at waste water treatment plants.
The invention is based on the fact that it is possible to prepare in advance an aldehyde-secondary amine adduct the stability and reactivity of which remain for quite a long period. Thus the handling of two difficult chemicals is avoided, and at the same time the number of work steps is reduced and it is possible to prepare a cationic (meth)acrylamide polymer suited specifically for a given treatment plant.
In accordance with the present invention, an aldehyde-secondary amine adduct prepared in advance is obtained by mixing an aqueous solution of a secondary amine with an aqueous solution of an aldehyde. The aldehyde may be formaldehyde, paraformaldehyde or 1,3,5-trioxane, preferably formaldehyde. The secondary amine may be any dialkylamine, the alkyl carbon chain of which may have 1-5 carbon atoms, the most preferable being dimethylamine. The mixing ratio of the aldehyde to the secondary amine may vary within the range 2:1-1:2. The reaction temperature may vary from room temperature to very high temperatures, depending on how rapid an adduct formation is desired. The formed aldehyde-amine adduct is stabilized to increase shelf life and resistance to reactions by adjusting the pH to 7 or below or by adding methanol to the mixture, or most preferably my adjusting the pH to 7 or below and by adding methanol. The adjustment of the pH may be carried out using either an organic or an inorganic acid, preferably hydrochloric acid, sulfric acid or oxalic acid. The adduct thus stabilized can be transported ready-made to the point of use, where the actual cationic modification of the (meth)acrylamide polymer is carried out to the desired degree of cation exchange capacity.
It is recommended that at the point of use the actual modification of the (meth)acrylamide polymer is carried out in a polymer dissolution apparatus. The adduct solution is added to the dissolution apparatus after the dissolution of the polymer or already during the dissolving. The reaction will progress to some degree at any pH, but in order to accelerate the reaction the pH of the polymer-adduct solution is adjusted to a sufficiently high value, typically above 9, with the necessary amount of an alkali, typically an alkali metal hydroxide or an alkali metal carbonate. The alkali is preferably sodium hydroxide, sodium carbonate, potassium hydroxide or potassium carbonate. The alkali may be added to the solution at the same time as the polymer or separately. The treatment may be carried out at various temperatures, typically at 20-80xc2x0 C. At a higher temperature the reaction time is shortened, being 10-15 hours at 20xc2x0 C., 2-5 hours at 40xc2x0 C., and 0.5-1 hour at 60xc2x0 C. Preferably polymer is dissolved in hot water in the dissolution apparatus in order that a temperature higher than room temperature should at the same time be obtained for the reaction.
The polymer used in the treatment may be polyacrylamide or polymethacrylamide, or a copolymer of acrylamide or methacrylamide with one or more monomers. Preferably the polymer is polyacrylamide or a copolymer of acrylamide with a cationic monomer. The polymer may be immediately soluble in water, or may become water-soluble only along with the treatment. The molecular weight of the polymer should in general be very high, typically above 100,000. For this reason the concentration of the polymer solution treated is low, typically below 5%, preferably below 1%. The degree of treatment, i.e. the molar ratio of the reacting amine groups and polymer amide groups used, may vary within the range 0.01-1.
The cationic polymer obtained from the treatment may be used, as can other cationic polymers, for example as a flocculant in the settling of colloidal fines, as a retention agent in paper making, for the dewatering of slurry in the treatment of waste waters, and as a filtration aid.
The invention is described with the help of the following examples, the purpose of which is not to limit the scope of the invention.